Prior art acoustic position locating systems include a variety of ultrasonic signal generating means. Some systems employ a pointer having an incorporated spark gap. The spark gap generates an acoustic signal that is propagated to orthogonally oriented, linear microphones. Other systems employ acoustic transducers that emit periodic, acoustic signals that are received by appropriately located, discrete microphones. In both such systems, circuitry is provided which measures the propagation time of the acoustic signal between the transmitter and the respective receiving microphones. The propagation time of each signal is then converted into distance to enable the location of the transmitter. An example of the latter type of ultrasonic position locating system can be found in U.S. patent application Ser. No. 07/412,885, filed Sept. 26, 1989 now U.S. Pat. No. 4,991,148 to the inventor hereof.
Detection of acoustic position determining signals has been performed using several techniques. Spark-acoustic positioning systems generally employ amplitude discrimination circuits that only become active when the sensed signal exceeds a preset amplitude. Such circuits are used in systems such as shown in Whetstone, et al. U.S. Pat. No. 3,838,212; Davis et al. U.S. Pat. No. 4,012,588; and with other spark-acoustic systems.
In Herrington et al. U.S. Pat. No. 4,654,648 , a stylus emits periodic acoustic signals, and a linear array of microphones receives those signals and determines the position of the stylus by hyperbolic triangulation. In that system, point source acoustic transmitters achieve uniform acoustic transmission patterns. The Herrington et al. detection circuit includes a zero crossing detector and also a sample and hold circuit for retaining the peak amplitudes of each cycle. A time latch is responsive to a determination by zero crossing detector that a signal has been received, to latch a clock whose count was commenced when the acoustic signal was first transmitted. Once a microprocessor receives a count from each receiver and the amplitude information for each received signal, the microprocessor scales the amplitude data and locates a common amplitude point on each of the received waves by correlation. The processor then determines the difference between the arrival times of the waveforms at the different receivers and determines the position of the transmitter.
In contrast to spark-gap/amplitude discrimination receiving circuitry, correlation circuitry provides an accurate signal representation and thus is able to precisely determine the time of signal arrivals. Such circuitry is expensive, complex, and somewhat slow in operation.
Accordingly, it is an object of this invention to provide simplified circuitry for detecting a received acoustic signal and to determine therefrom, the distance of an acoustic transmitter from an acoustic receiver.
It is another object of this invention to provide an acoustic signal detection circuit that provides the signal detection accuracy of a correlation circuit, while exhibiting less complexity and cost.